Labeled a "salacious teenage image of the future princess," the photo also contains the words "not to be published" emblazoned on its front, possibly explaining how it never appeared in the Mirror, but not why.

The tabloid, which purchased the snapshot two days before Diana's engagement to Prince Charles in February 1981, eventually sold the 10 x 8" glossy news photo along with other archival photos to the Caren Archive, which in turn put it up for auction.

Because of its mysterious nature, the photo is expected to sell for well over $1,000 when bidding ends on January 24th.

UPDATE: Diana's "mystery man" has been identified as Adam Russell, "the great-grandson of former prime minister Stanley Baldwin, and now a deer farmer in Dorset."

The two were both injured during a skiing vacation and "kept each other company... but absolutely nothing happened."